She was awfully defensive, I think she would have insisted on going back for her backpack despite what she said.
Well, at first I also thought so, but Mallory mentioned in a couple of interviews that when she raced with her dad, they had already decided that a lost backpack would be left behind in case that ever happened, so they could race on. And that was also why Gary & Mallory always kept all of their necessary stuff (medicine, passports etc.) in one backpack, so they would have less of a chance to lose those.
... and they never had the time to talk such things with Mark :/